The invention relates generally to exciter circuits for ignition systems used with internal combustion engines. More particularly, the invention relates to exciter circuits that utilize solid-state switches such as, for example, thyristors, as control devices for producing sparks.
A conventional ignition system for an internal combustion engine, such as, for example, a gas turbine aircraft engine, includes a charging circuit, a storage capacitor, a discharge circuit and at least one igniter plug located in the combustion chamber. The discharge circuit includes a switching device connected in series between the capacitor and the plug. For many years, such ignition systems have used spark gaps as the switching device to isolate the storage capacitor from the plug. When the voltage on the capacitor reaches the spark gap break over voltage, the capacitor discharges through the plug and a spark is produced.
More recently, turbine engine and aircraft manufacturers have become interested in replacing the spark gap with a solid-state switch, such as an SCR-type thyristor. This is due, in part, because an SCR typically operates longer than a spark gap tube which may exhibit electrode erosion. Also, SCR switches are produced in large volume making them less expensive than spark gaps which are individually crafted in small quantities. Furthermore, the storage capacitor's voltage at discharge remains essentially constant over the life time of the SCR switch, but can change significantly during the life of the spark gap due to electrode erosion.
However, there are also significant disadvantages to replacing a spark gap with a conventional SCR switch. One concerns the peak power produced by the spark discharge pulse. Although spark energy is about the same for the spark gap and SCR switch designs, peak spark power is severely reduced using known SCRs because these device are limited to peak discharge currents of about 1000 amps with a current transition rate (i.e. di/dt) limit of about 200 amps/.mu.second. In contrast, spark gap discharge currents rise rapidly at about 1000 amps/.mu.second to a peak of about 2000 amps. This produces a high peak power that causes a loud bang and sonic shock wave that emanates from the igniter tip. It is this shock wave that breaks up and disperses the fuel particles making them easier to ignite. The high peak current and current transition rates required for high peak power do not present a problem for spark gaps but are of a destructive nature for conventional SCR devices.
When a conventional SCR is gated on, initially only a very small portion of the die area around the gate electrode attachment conducts current due to a finite spreading velocity. If a fast rising current is permitted at turn on, a high current density occurs in the small conducting area of the die resulting in high switching losses. These high losses create excessive heating and are of a destructive nature to the SCR device. To allow proper current spreading of the entire die area which will permit a safe operating environment for the SCR, a saturable core inductor, often referred to as a delay reactor, must be incorporated in the circuit design. The delay reactor is connected in series with the SCR switch, and the inductance of the reactor limits the rate of rise of the current (di/dt) for a period of time while the SCR is turning on. Once the SCR is in full conduction, the delay reactor's core saturates and the inductance becomes so small that it no longer affects the circuit operation.
If too high a di/dt level is applied to a conventional SCR device, the device will eventually and gradually become leaky, and a reduction in the break over voltage will slowly occur. The rate at which these changes take place is dependent upon how high the di/dt levels are that the switching device experiences over time.
Based on testing that has been conducted by engine manufacturers on ignition systems that employ solid-state technology, ignition lightoff has been a problem and a concern. It is believed that these no lightoff conditions are caused by at least two characteristic differences. One is that the reduced peak power level is not sufficient to maintain a clear plug, thereby resulting in the absence of a spark due to contamination fouling. The second condition results in less of a shock wave being developed, as a result of the peak power reduction, which may not be sufficient for igniting the fuel particles under more severe fuel-air ratios and contaminated mixtures.
Other disadvantages result from the SCR being a regenerative type of switch. When conduction current in a regenerative switch exceeds a critical latching level it acts as a source of internal control current sufficient to maintain the switch in conduction even after the external gate control signal is removed. As conduction current increases so does the internal control current and in effect the conduction current drives and latches the device on. This regenerative action enables the SCR to conduct the high peak currents required of exciter circuits but it also creates problems when the SCR is required to turn off or block current.
Leakage current in conventional SCR devices increases significantly at high operating temperatures. When the leakage current of an off state SCR exceeds the critical leakage level, the SCR, without an external gate signal to initiate conduction, will turn on and stay on. For this reason SCR junction temperatures cannot typically be operated above 150.degree. C. where uncontrolled turn on renders them useless. Non-regenerative semiconductor switches such as FET devices and transistors typically operate as junction temperatures of 200.degree. C. and above.
Even when leakage current is not sufficient to turn an SCR on it can be high enough to act as a load on the exciter's charging circuitry and divert charging current away from the storage capacitor. This causes the spark rate to decrease. To maintain a constant spark rate, known exciter designs must utilize additional timing and regulating circuitry to compensate for the leakage problem. Furthermore, the charging currents for the main storage capacitor can exceed the sustaining current limits for a conventional SCR, thus tending to keep the SCR switched on (conducting) after the capacitor has discharged through the device. Because of this problem, special circuitry is required to either turn off the charging current for a short time period or to otherwise commutate the SCR devices so as to allow the devices to recover and turn completely off so as to block voltage during the succeeding charging period. The special charging interrupt circuits can prevent direct drop-in replacement of an SCR exciter for a spark gap exciter during maintenance and overhaul.
Thus, there is a need for a simple and reliable exciter, preferably using high-temperature solid-state switches, that produces high energy sparks with high peak power at a stable spark rate without switch degradation and that can be a direct replacement for conventional spark gap exciters.